1. Field of the Invention
This invention generally relates to an improved heater assembly and method for applying a brazing heat across a selected longitudinal section of a tube. The invention is particularly useful in brazing reinforcing sleeves within the heat exchange tubes of nuclear steam generators.
2. Description of the Prior Art
Heater assemblies for applying a brazing heat to the inside surfaces of conduits are known in the prior art. Such heaters are used to braze reinforcing sleeves within the heat exchange tubes of nuclear steam generators damaged by corrosion. In such prior art processes, a tubular sleeve which is circumscribed at each end by a ring of brazing alloy is inserted into a tube in need of repair and slidably positioned across the section of the tube where the walls are in a damaged condition due to corrosion or denting. The ends of the tube surrounded by the rings of brazing alloy are then usually hydraulically expanded so that they snugly engage the inner walls of the tube. To complete the joint, a brazing heater assembly having an electrical resistance wire coiled therearound is slid up the sleeve and actuated so that the heat generated by the wire melts the rings of brazing alloy disposed between the outer wall of the sleeve and the inner wall of the tube, thereby forming a pair of watertight braze joints between the ends of the sleeve and the tube. Such prior art sleeving operations are frequently performed in the longitudinal sections of the heat exchange tubes which extend through the tubesheet of the nuclear steam generator due to the tendency of the walls in these tubes to corrode and dent in the region. Specific examples of such heater assemblies are set forth in U.S. patent application Ser. Nos. 571,214 and 571,215, filed on Jan. 16, 1984 by John M. Driggers and John M. Driggers et al, respectively, and assigned to Westinghouse Electric Corporation. While such heater assemblies are capable of producing high-quality braze joints between corrosion-damaged heat exchange tubes and the ends of the reinforcing sleeves used to repair these tubes, such heaters are not entirely without shortcomings. For example, the useful service life of the heater mandrel is often unsatisfactorily short due to the tendency of the mandrel material (preferably boron nitride) to oxidize and "burn out" in response to the heat generated by the electrical heating element coiled therearound. Additionally, since the various components forming such heater assemblies are permanently attached to one another, the burning out of the heater mandrel usually necessitates the disposal of the entire heater assembly. Finally, such heater assemblies occasionally transfer heat to the inside walls of the tube through a non-uniform heat flux due to the lack of any positive centering force on the heater mandrel. This deficiency can have at least two serious consequences. First, such non-uniformities in the heat flux might leave a portion of the ring of brazing alloy incompletely fuxed, thereby resulting in a poor-quality braze joint. Second, if the operator of the heater assembly is attempting to utilize a temperature sensor in conjunction with the heater assembly to confirm whether a proper amount of brazing heat has been applied to the inside surface of the reinforcing sleeve, such non-uniformities can lead to inaccurate temperature readings. Such a combination heater assembly and temperature sensor is described and claimed in U.S. patent application Ser. No. 720,105, filed Apr. 4, 1985 entitled "Radiant Brazing Temperature Sensing Apparatus and Process", by John M. Driggers and assigned to Westinghouse Electric Corporation, the entire specification of which is expressly incorporated herein by reference.
Clearly, there is a need for a heater assembly having a means for eliminating (or at least reducing) the oxidation of the heater mandrel caused by its contact with the electrical resistance heating element. Ideally, such a heater assembly should also have some sort of means for positively centering the heater mandrel so that heat is transferred in a uniform pattern from the coil of electrical resistance wire to the inner walls of the sleeve. Such a positive centering force would have the additional beneficial effect of preventing the coil windings from scraping against the inner surface of the sleeve, which in turn preserves the coil from damage. Finally, it would be desirable if the various major components of the heater assembly were detachable, so that major components which wear out may be conveniently replaced.